Biobutanol produced by a microorganism of the genus Clostridium has attracted great attention as post-bioethanol. As biobutanol production by a microorganism of the genus Clostridium also produces acetone and some ethanol simultaneously, it is also called acetone-butanol-ethanol fermentation (ABE fermentation). Compared with ethanol, butanol is advantageous in terms of energy density and physical properties such as low corrosiveness and hygroscopicity. Further, butanol not only can be used as a raw material of a chemically-synthesized substance such as isoprene, isobutene and butane, but also can be added to diesel fuel as well as gasoline fuel; butanol has a wide variety of uses.
On the other hand, ABE fermentation has a complex and unique metabolic pathway in which a metabolite is changed significantly depending on the state of bacteria (Non-patent Document 1). Further, since butanol inhibits growth of a production microorganism, fermentation productivity of butanol is significantly lower than that of ethanol, and there exist many technical problems which must be solved in order to improve productivity. Regarding biobutanol production, the present inventors have reported a system of production by pH-stat fed-batch culture with growing cells using butyric acid as a substrate (Non-patent Document 2), a highly-efficient system of production with resting cells (Non-patent Document 3) and the like.
On the other hand, the use of edible biomass such as corn or sugarcane as a raw material to produce biofuel or biomaterial by a microorganism has a problem that it will create a competition between biofuel and food-production, which leads to an increase in price. As a solution for this problem, production of a substance using inedible biomass as a raw material has drawn attention. While edible biomass contains a large amount of amylose and thus can easily be fermented, inedible biomass in general contains large amounts of cellulose and hemicellulose. Hence, the following approaches have been taken in the research on the use of inedible biomass: decomposition of cellulose or hemicellulose by saccharogenic amylose or the like into glucose or xylose, which can be used as a substrate fermentable by a microorganism; and acquisition of a microorganism that can use cellulose or hemicellulose directly as a substrate.
However, direct use of inedible biomass as a raw material in fermentation to produce biobutanol has not been accomplished.